1. Field of the Invention
The present invention relates to a method of producing a metal oxide thin film using a coating method, the metal oxide thin film being used as a semiconductor layer, an insulating layer, or a conductive layer in various electronic devices such as a thin film transistor. Further, the present invention relates to a coating solution used in the same method and a metal oxide thin film obtained using the same method.
2. Description of the Related Art
A thin film transistor is mounted in an electronic device such as a liquid crystal display, an organic EL display, or an X-ray sensor. As a substrate for forming a thin film transistor, in general, a glass substrate is used. However, an electronic device is required to be flexible and be reduced in weight, and the use of a flexible and light-weight substrate is desired.
Examples of the flexible and light-weight substrate include: a resin film such as a plastic substrate; and a flexible substrate such as a thin metal substrate. From the viewpoints of transparency, substrate price, and the like, an inexpensive plastic substrate is preferable. Since an inexpensive plastic substrate has a problem in heat resistance, it is necessary to form a thin film transistor through a low-temperature process.
In an electronic device such as a thin film transistor, a metal oxide thin film is used as a semiconductor film such as an active layer, an insulating film, or a conductive film such as an electrode.
In general, a metal oxide thin film is formed using vacuum deposition such as a sputtering method. As a method of simply forming a metal oxide thin film at a low temperature under the atmospheric pressure, a film formation method using a liquid-phase process is considered.
Nature, Vol. 489, p. 128, 2012, reports a method of manufacturing a thin film transistor (TFT) having high charge transport characteristics at a low temperature of 150° C. or lower, the method including: coating a substrate with a solution containing a precursor of a metal oxide semiconductor thin film; and performing ultraviolet irradiation.
In addition, a method of obtaining a metal oxide semiconductor film is reported, the method including: forming a metal oxide semiconductor precursor film using a solution containing an inexpensive nitrate, acetate, or the like; and performing a semiconductor conversion treatment on the precursor film (for example, WO2009/081862A, WO2009/031381A, WO2009/011224A, and JP2010-258057A).
WO2009/081862A discloses a method of producing a metal oxide semiconductor film, the method including: forming a precursor film containing a nitrate or an acetate of In, Zn, Sn, Ga, Al, or the like; and performing a semiconductor conversion treatment such as a heat treatment at 100° C. to 400° C. or microwave irradiation on the precursor film.
WO2009/031381A describes that electromagnetic irradiation is performed in the presence of oxygen as a semiconductor conversion treatment. WO2009/011224A describes that ultraviolet irradiation is performed in the presence of oxygen as a semiconductor conversion treatment after a solvent is volatilized by heating at about 150° C. JP2010-258057A describes that a coating film is dried by volatilizing a solvent at about 80° C. to 100° C., is heated at 50° C. to 200° C., and is converted into a metal oxide semiconductor using an oxygen plasma method, a thermal oxidation method, a UV ozone method, or microwave irradiation (refer to JP2010-258057A).